A number of optical applications in the deep ultra violet (DUV) range have limitations due to the absence of simple and reliable optical notch filters. A “notch” filter is one that blocks a narrow wavelength region and transmits other wavelengths.
This is particularly limiting since the DUV optical range is an interesting range and fruitful for a number of remote sensing applications that take advantage of “solar blind measurements”. e.g. as described in M. Razeghi, “Short-Wavelength Solar-Blind Detectors—Status, Prospects and Markets” Proceedings of IEEE V. 90, No 6, pp. 1006-1014 (2002) where relatively high atmospheric transmission is combined with virtually no solar background light (blocked by ozone layer in upper atmosphere).
Such a filter could be used to suppress elastically scattered light (Raleigh or diffuse scattering from a sample) in ultra-violet resonance Raman (UVRR) diagnostics, e.g. as described in S. A. Asher, “UV Resonance Raman Spectroscopy for Analytical. Physical and Biophysical Chemistry”, Analytical Chemistry V. 65. No 2. pp 59A 66A, 201A-210A (1993). One such diagnostic, Swept Wavelength Optical Resonance Raman Diagnostic (SWORRD), is described in Grun et al., U.S. Pat. No. 7,436,510, issued Oct. 14, 2008, and incorporated herein by reference. An effective notch filter would allow the use of compact single-stage spectrometers for data acquisition. However, a limited choice of optically transparent materials in the DUV range is a considerable obstacle to the design of interferometric filters or acousto-optical tunable notch filters for wavelengths shorter than 300 nm.
A birefringent filter design originally proposed by {hacek over (S)}olc, described in 1. {hacek over (S)}olc “Birefringent chain filters” JOSA V. 55, p. 621 (1965), and incorporated herein by reference, is suitable for DUV applications. Such {hacek over (S)}olc filter includes two typical configurations, fan and folded. A detailed description of both types of {hacek over (S)}olc filters and analysis of their spectral transmission can be found in A. Yariv and P. Yeh, “Optical Waves in Crystals” Wiley Classics Library Edition, ISBN 0-471-43081-1, pp 133-154 (2003), which is based on Jones matrix formalism, described in R. C Jones, “New calculus for the treatment of optical systems”, JOSA V. 31, p. 488 (1941). These designs, however, are exclusively for narrow-line transmission (<1 nm), rather than for rejection (or blocking) of narrow linewidths.
It would be desirable to provide an optical filter to remove these limitations.